What Controls the Martian Top-Side Ionosphere? An Analysis of Balance in Solar Wind Induced Magnetic Pressure and Cold Thermal Ionospheric Pressure at Mars

Electromagnetic forces in the Martian ionosphere produce sufficient pressure to deflect the solar wind. Magnetic pressure often dominates the pile-up region of magnetic fields from the solar wind, which resides above the ionosphere. Below this region, thermal pressure increases and becomes the dominant pressure, marking the transition to the top-side ionosphere. We present a study of this transition region using measurements from the Langmuir Probe and Waves (LPW) and magnetometer (MAG) instruments on the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter. We constrain this study to a dayside altitude range from 180 km, where ions are demagnetized, to 800 km, which roughly corresponds to the lower magnetosheath boundary. After identifying the transition points, we investigate controlling factors such as solar-zenith angle, geographical location, and in situ plasma/fields properties. The quantity of transition points over an orbit leg can indicate particular solar wind or Martian ionosphere conditions. We expected most orbits to have only one transition from thermal pressure at lower altitudes to magnetic pressure at higher altitudes. However, we find that only a minority of orbits can be classified in this manner; instead, most orbits exhibit multiple transitions. Near noon, these single transition regions are located mainly below altitudes of 400 km and decreasing in altitude with stronger magnetic field strength. The single transition regions are mainly found outside of areas with crustal fields. Preliminary results indicate that cases with no transitions observed often occur on orbits where the periapsis of MAVEN resides in the southern hemisphere. Using these observations, we report on the categories of transition regions to infer the structure of the Martian ionosphere.